Hybrid, fuel cell, and electric vehicles often use high voltage circuits to provide power to electric drive motors and for various other uses on the vehicle. High voltage electricity can potentially cause an electric shock hazard to any user or mechanic able to come in contact with it. One common method of reducing the risk of electric shock hazard is by using an automatic disconnect device.
An automatic disconnect device is generally implemented as a pair of high voltage relays with associated control circuits. It is placed electrically between the string of batteries in a battery pack and the traction components in the vehicle. The high voltage relays, which are also called contactors, can limit the electrical energy to one electrical side of the automatic disconnect device in response to various faults on the vehicle. Once opened, the high voltage electrical energy is confined to the battery pack.
High voltage components often use large capacitors to buffer their energy usage and provide quick bursts of energy. These capacitors are charged to full voltage during operation. Under some fault scenarios the capacitors are not discharged even after the high voltage relays are opened. Because of this, most vehicle manufactures install passive discharge resistors in their systems near the capacitors. Because passive discharge resistors take a significant amount of time to discharge the full capacitance, some manufactures also include an active or automatic discharge function with the automatic disconnect device.
The main difficulty with the traditional approach of implementing the automatic discharge function is that the control of the function is very complex. For example, the controlling transistor must not be turned on while the battery pack is still connected or the discharge resistor could be damaged by overloading. In some fault situations, such as during a loss of electrical power while controlling a permanent magnet motor, the transistor should generally be turned on automatically. Generally, a significant portion of this complex control is directed by software within the vehicle controllers. Because it is under software control it is somewhat more prone to incorrect activations, either not activating when it should or activating when it should not.
Accordingly, it is desirable to have a simple system for automatic discharge. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.